For a given airspeed, an aircraft may consume less fuel at a higher altitude than it does at a lower altitude. In other words, an aircraft may be more efficient in flight at higher altitudes as compared to lower altitudes. Moreover, bad weather and turbulence can sometimes be avoided by flying above such weather or turbulence. Thus, because of these and other potential advantages, many aircraft are designed to fly at relatively high altitudes.
As the altitude of an aircraft increases, from its take-off altitude to its “top of climb” or “cruise” altitude, the ambient atmospheric pressure outside of the aircraft decreases. Thus, unless otherwise controlled, air could leak out of the aircraft cabin causing it to decompress to an undesirably low pressure at high altitudes. If the pressure in the aircraft cabin is too low, the aircraft passengers may suffer hypoxia, which is a deficiency of oxygen concentration in human tissue. The response to hypoxia may vary from person to person, but its effects generally include drowsiness, mental fatigue, headache, nausea, euphoria, and diminished mental capacity.
Aircraft cabin pressure is often referred to in terms of “cabin altitude,” which refers to the normal atmospheric pressure existing at a certain altitude. Studies have shown that the symptoms of hypoxia may become noticeable when the cabin altitude is above the equivalent of the atmospheric pressure one would experience outside at 8,000 feet. Thus, many aircraft are equipped with a cabin pressure control system to, among other things, maintain the cabin pressure altitude to within a relatively comfortable range (e.g., at or below approximately 8,000 feet) and allow gradual changes in the cabin altitude to minimize passenger discomfort.
Some cabin pressure control systems implement control logic that may, when needed or desired, begin pressurizing the aircraft cabin (or “descending” the aircraft cabin) before take-off, either while taxiing on or to the runway or at the start of the take-off roll down the runway. This initial cabin pressurization process is sometimes referred to as “cabin pre-pressurization.” The cabin pre-pressurization process, when implemented, is preferably initiated and conducted at a pressurization rate (or “descent rate”) that will not cause passenger discomfort. Various standard setting organizations within the aerospace industry have established −300 sea-level-feet-per-minute (slfpm) as the preferred pressurization rate (or descent rate limit). In attempts to quickly achieve this preferred pre-pressurization rate, many current cabin pressure control systems implement pre-pressurization control logic that commands a cabin pressurization rate (descent rate limit) to an artificially high pressurization rate for a pre-determined time period.
Although the above-described cabin pre-pressurization control logic is generally safe, robust, and effective in quickly achieving the preferred cabin pressurization rate, it can exhibit certain drawbacks. For example, the cabin rate response can result in either a rate overshoot, which can lead to potentially uncomfortable cabin pressurization rates, or a rate undershoot, which can lead to insufficient pre-pressurization performance. This rate control inconsistency may be most pronounced with variations in aircraft characteristics, such as cabin air inflow and pressurized volume, which can occur on a flight-by-flight basis. Because these aircraft characteristics can vary significantly, this can lead to inconsistent pre-pressurization performance and customer dissatisfaction.
Hence, there is a need for a cabin pressure control system and method that controls cabin pressurization rate to quickly and consistently pressurize an aircraft cabin at a rate that does not cause passenger discomfort and/or dissatisfaction, and/or at a rate that does not significantly vary with variations in aircraft characteristics. The present invention addresses one or more of these needs.